1. Field of the Invention
The present invention relates to a receiving circuit of a cellular telephone set operating on both the code division multiple access method and the frequency division multiple access method. More particularly, the invention relates to an intermediate frequency circuit in the receiving circuit.
2. Description of the Related Art
Described below with reference to FIG. 2 is part of a conventional receiving circuit used by what is known as a dual mode cellular telephone set operating on both the code division multiple access method and the frequency division multiple access method.
In a cellular telephone set 31 shown in FIG. 2, a signal from a transmitting circuit 32 is fed through an antenna sharing device 33 to an antenna 34 from which the signal is transmitted to a base station, not shown. A signal sent from the base station is received by the antenna 34 and input to a receiving circuit 35 via the antenna sharing device 33. This type of cellular telephone set 31 is capable of sending and receiving signals to and from the base station in either a code division multiple access mode (called the CDMA mode hereunder) or a frequency division multiple access mode (called the FM mode hereunder).
In the receiving circuit 35, a received signal on a bandwidth of about 880 MHz passes through the antenna sharing device 33 to reach a low-noise amplifier 36 for amplification. The amplified signal is input to a mixing circuit 37 whereby the signal is mixed with an oscillation signal from a local oscillator 38. The mixing circuit 37 outputs a signal with an intermediate frequency of about 85 MHz to an intermediate frequency amplifier 39. The amplifier 39 converts the frequency of the received signal to a bandwidth per channel of about 1.23 MHz for the CDMA mode or about 30 kHz for the FM mode. The received signal of either the CDMA or FM mode alone is converted in terms of frequency.
The intermediate frequency amplifier 39 uses a transistor 42 whose base is supplied with a fixed bias voltage from bias resistors 40 and 41. An intermediate frequency signal, after being amplified to the appropriate level by the intermediate frequency amplifier 39, is output from the collector of the transistor 42.
Following amplification by the intermediate frequency amplifier 39, the intermediate frequency signal of the CDMA mode is sent through a switching diode 43 to a filter 45 dedicated to filtering CDMA mode intermediate frequency signals; the intermediate frequency signal of the FM mode is forwarded through a switching diode 44 to a filter 46 that filters FM mode intermediate frequency signals.
Where the cellular telephone set 31 is used in the CDMA mode, a mode switching voltage applied to a mode switching terminal X1 causes a current to flow through a resistor 47, the switching diode 43 and a resistor 48. This causes the switching diode 43 to conduct, allowing the CDMA mode intermediate frequency signal to reach the filter 45. Where the cellular telephone set 31 is operated in the FM mode, a mode switching voltage applied to a mode switching terminal X2 causes a current to flow through a resistor 49, the switching diode 44 and the resistor 48. This causes the switching diode 44 to conduct, carrying the FM mode intermediate frequency signal to the filter 46.
Different levels of electrical performance are required in each of the CDMA mode and the FM mode. The operation in the CDMA mode, in particular, requires that any intermodulation distortion caused by the intermediate frequency amplifier 39 be minimized. That requirement is met typically by establishing values of the bias resistors 40 and 41 such that a large operating current (i.e., collector bias current) flows continuously through the transistor 42 of the intermediate frequency amplifier 39. That is, the intermediate frequency amplifier 39 is made to operate from the same operating current regardless of the CDMA mode or FM mode being in effect.
In the conventional receiving circuit of the conventional cellular telephone set, as described, a large operating current flows through the transistor 42 of the intermediate frequency amplifier 39. It should be noted here that characteristics required of the intermediate frequency amplifier 39 in the FM mode in connection with intermodulation distortion are not as strict as in the CDMA mode, since the FM mode involves frequency modulation. That is, the conventional receiving circuit dissipates more power than is necessary while the FM mode is being selected. This tends to promote dissipation of cells and render the cellular telephone set less convenient than it should be.